The objectives of the proposed research are to study various factors which may affect the size of a neuron population during normal development. A prevailing explanation for the precise control of neuron numbers in that neurons and their targets are matched by a process involving competition between neurons and selective neuronal cell death. In the African clawed frog, Xenopus laevis, individual variation in the number of motoneurons in the lumbar lateral motor column is highly correlated with body size in both naturally occurring diploid and experimentally produced triploid animals. Whether this size-related variation in neuron number is produced by a competition process involving some feature of the hindlimb muscles innervated by these motoneurons is unclear since the significant correlation between motoneuron number and muscle fiber number observed in diploid animals is not observed in their triploid siblings. However, primary myotubes, a precursor of muscle fibers, may be the relevant feature of the target that limits motoneuron survival in both diploids and triploids. Correspondences between the average number of primary myotubes and the average number of motoneurons after cell death have been noted by other investigators, but the relationship between motoneuron number and myotube number in individual animals has never been determined. Two sets of experiments using Xenopus siblings are proposed to clarify the role that the primary myotubes may have in limiting motoneuron survival during normal development. First, after determining when primary myotube formation in a representative hindlimb muscle is complete, both the myotubes and the motoneurons in the same animals will be counted to measure the individual variation in these cell populations and to determine how the populations are related before motoneuronal cell death occurs. Second, because the free-living Xenopus larvae grow normally following hindlimb amputation, the myotubes in the muscle of an amputated limb will be counted and compared with the number of motoneurons (identified by retrograde labelling) innervating the muscle of the remaining limb after cell death. The hypothesis that myotube number is specifically relevant to motoneuron survival during normal development can be tested. In general, the information is relevant to understanding what processes are likely producing the natural variation in neuron number and the striking bilateral symmetry in neuron number observed after normal development.